1. Field of the Invention
The present invention relates to methods and apparatus for generating multi-beam, multi-signal transmissions using an active phased array antenna.
2. Description of the Related Art
There are many communications, radar, and navigation applications that require multiple transmit beams from a common site, with different signals on the various beams. For example, the next generation of Global Positioning System (GPS) satellites will require the transmission of spot beams to direct higher power military signals to theaters of operation, along with broader earth coverage beams for conventional navigational users. Proposed solutions include the use of a separate gimbaled antenna to provide the spot beam, in addition to the existing earth coverage antenna. Alternative approaches using an active phased array antenna with an electronically steered beam to implement the spot beam have also been proposed.
The use of a separate antenna (mechanically steered or electronically steered) for each beam also requires the use of a separate transmitter and associated electronics to drive each antenna. This greatly increases system cost, size, weight and power requirements over what would be required for a single phased array antenna and transmitter. For example, separate High Power Amplifiers (HPAs) would be required to drive each antenna. Also, the use of separate antennas and separate transmitters for the various beams (e.g., earth coverage and spot beams) limits the ability to rc-allocate power among the various beams. This flexibility would provide significant benefit for many applications. For example, the ability to re-allocate power among beams would provide more power to support additional missions with the earth coverage beam when the spot beams are not in use—a very desirable feature for future versions of GPS.
Note that active phased array antennas have been configured to provide multiple simultaneous beams with the same signal on all of the beams. However, a phased array configuration with different signals on each of the beams has not previously been implemented. Preferably, a system capable of producing multiple beams for multiple signals would permit the use of composite signals whose amplitude envelopes are constant. If non-constant envelope signals are applied to the HPAs, the use of highly efficient, saturated HPAs is precluded. Linear methods that generate non-constant-envelope composite signals result in power-inefficient mechanizations, because the power amplifiers that are used for transmission of the composite signals must operate in the linear region. Power amplifiers are much more efficient when operated in the saturated mode. For example, linear superposition of chip-synchronous, orthogonal signals to be transmitted is a theoretically lossless multiplex if the subsequent transmission chain remains linear. Maintaining linearity requires a linear high power amplifier (HPA). Since any HPA characteristic eventually saturates as its input power increases, such base station transceiver linear amplifiers are typically run at 4-5 dB average power backoff to accommodate peak power needs.
Thus, linear combination techniques are maximally efficient in the sense that there is no actual signal power loss, but the overall efficiency of such techniques is compromised by the need to operate the amplifier at a significant power back-off to accommodate the instantaneous signal envelope fluctuations. An alternative approach to producing greater average power is to achieve a more effective allocation of the loss budget between the multiplexer and the high power amplifier. Non-linear multiplex methods that produce a constant-envelope composite signal permit a greater fraction of the available transmitter power to be used for communication, but at the expense of a multiplexing loss that may be characterized as an intermodulation product. This multiplexing loss, however, is typically smaller than the power backoff it replaces, resulting in a favorable trade. Therefore, constant-envelope signal structures are required if full-power, undistorted transmission is sought. Consequently, in developing a scheme to simultaneously transmit multiple beams with multiple signals, it would be desirable to transmit from each antenna element in the array a constant amplitude envelope composite signal to permit the use of power-efficient, saturated HPAs.